In recent years, digital content such as digitized audio and video data has been prevalent in great numbers. The digital content is easy to provide a faithful reproduction of original content, and it is an issue to protect the copyright of the digital content.
A “digital watermark” is used as one of countermeasures to protect the copyright of the digital content. The digital watermark is an art operable to embed data into image data in such a manner as to prevent human beings to perceive degradation in image, and operable to detect the embedded data.
Reference No. 1 (“The Basic of Digital Watermarks” by Koushio MATSUI, pages 91-94, published by Morikita Shuppan, 1998-edition) discloses a statistic-based patchwork system as an example of the digital watermark. The patchwork system is now described with reference to FIG. 11. FIG. 11 is a pattern diagram showing a prior art digital watermark system.
Initially, the following discusses the embedment of additional information. As illustrated in FIG. 11, any two pieces of pixel data (ai, bi) included in image data are selected as a data set. The two pieces of pixel data as included in the data set are changed in data value, thereby embedding additional information into the image data.
The additional information is one-bit information having a value of either “1” or “0”. One of the two pieces of pixel data (ai, bi) is increased in data value by any amount of α (α>0) in accordance with the value of the one-bit information, while the other is decreased in data value by the amount of a When the additional information includes multi-bit information, then several data sets are specified from image data, thereby repeating the same processing at each of the data sets for each piece of one-bit information. When the one-bit information embedded in one of the data sets has the value of “1”, then the pixel data “ai” is increased by the amount of α, but the pixel data “bi” is decreased thereby. Conversely, for one-bit information having the value of “0”, the pixel data “ai” is decreased by the amount of α, but the pixel data “bi” is increased thereby.
In this way, the additional information is embedded into the image data. Alternatively, a piece of bit information, which forms part of the additional information, may be embedded into several data sets.
The following discusses the detection of the additional information.
To detect the additional information, statistic characters of nature images are used. Initially, in image data free of the embedded additional information, a value of a difference between two pieces of pixel data (ai, bi) included in a data set is calculated. At this time, all values of differences between the pixel data (ai, bi) in all of the data sets are calculated to provide a sum “Sn” of all of difference values. (Equation 1) is an expression to calculate the sum “Sn” of the difference values.
                              S          n                =                                            ∑                              i                =                1                            n                        ⁢                          (                                                a                  i                                -                                  b                  i                                            )                                ≈          0                                    (                  Equation          ⁢                                          ⁢          1                )            
As described by (Equation 1), an increase in number “n” of the data sets allows the sum Sn to approach the value of “0”.
Now, assume that one-bit information having the value of “1” is embedded into the pixel data (ai, bi) included in each of the data sets, thereby providing pixel data (ai′, bi′). (Equation 2) illustrates an expression to calculate a sum Sn′ of values of differences between several pieces of the pixel data (ai′, bi′).
                                                                        S                n                ′                            =                                                ∑                                      i                    =                    1                                    n                                ⁢                                  (                                                            a                      i                      ′                                        -                                          b                      i                      ′                                                        )                                                                                                        =                                                ∑                                      i                    =                    1                                    n                                ⁢                                  {                                                            (                                                                        a                          i                                                +                        α                                            )                                        -                                          (                                                                        b                          i                                                -                        α                                            )                                                        }                                                                                                                                          =                                                            ∑                                              i                        =                        1                                            n                                        ⁢                                          {                                                                        (                                                                                    a                              i                                                        -                                                          b                              i                                                                                )                                                +                                                  2                          ⁢                          α                                                                    )                                                                      }                            ≈                              2                ⁢                α                ⁢                                                                  ⁢                n                                                                        (                  Equation          ⁢                                          ⁢          2                )            
As described by (Equation 2), an increase in number “n” of the data sets allows the sum Sn′ to approach the value of 2αn. This means that a change amount α is detected, which has been imparted to the pixel data (ai, bi) to embed the additional information. In this instance, the calculated 2αn is a positive value, and the embedded one-bit information is determined as having the value of “1”. Conversely, when the one-bit information having the value of “0” is embedded, then the sum Sn′ approaches a negative value of “−2αn”. In this way, the value of the embedded bit information is determined based on either the negative or the positive of the sum Sn′ of values of differences between several pieces of the pixel data (ai′, bi′) included in all of the data sets that have the additional information embedded therein.
Reference No. 2 (published Japanese Patent Application Laid-Open No. 2000-228720) discloses another art for the detection of the additional information.
FIG. 12 is a pattern diagram illustrating the determination of additional information, as disclosed by Reference No. 2.
According to Reference No. 2, when one-bit information of additional information is embedded in a piece of pixel data, which forms part of image data, then the additional information is detected based on both of the pixel data having the additional information embedded therein, and neighboring pixel data in the neighborhood thereof.
As illustrated in FIG. 12, the additional information includes fifty-bit information, in which each bit is embedded at a corresponding spot of positions Nos. 1 to 100. The first to fiftieth bit are embedded at respective spots of positions No. 1 to No. 50, and at those of positions No. 51 to No. 100. More specifically, the first bit of the additional information is embedded at two spots, i.e., positions No. 1 and No. 51.
In short, the additional information is detected based on pixel data at position No. 1 and neighboring pixel data in the neighborhood of the pixel data at position No. 1, and pixel data at position No. 51 and neighboring pixel data in the neighborhood of the pixel data at position No. 51. The detection is repeated until the last position No. 100 is reached, thereby detecting the additional information that includes the fifty-bit information.
However, a problem with the information detection according to Reference No. 1 is that it is difficult to detect the change amount with accuracy because the sum Sn′ does not approach the value of either “2αn” or “−2αn” when there is a small correlation between the two pieces of pixel data (ai, bi) having the additional information embedded therein. The inaccurate detection of the change amount results in inaccurate detection of the additional information as well. Another problem incident to Reference No. 1 is that additional information having only very small capacity can be embedded into a piece of image data because a value of the number of “n” must be increased to detect a change amount “α” precisely.
Meanwhile, according to the information detection as disclosed by Reference No. 2, a gang of change amounts must be embedded into a piece of pixel data. Such a requirement forces the piece of pixel data to be considerably changed in data value, with a concomitant problem that images are susceptible to degradation.
In view of the above, an object of the present invention is to provide an information-detecting apparatus and method operable to detect additional information with increased accuracy from digital data having a large amount of additional information embedded therein, while avoiding degradation in images.